Colored photosensitive glasses or polychromatic glasses, as such products have more recently been termed, had their genesis in U.S. Pat. No. 4,017,318. Colors encompassing the entire spectrum of visible coloration can be developed within a single glass composition following that inventive concept.
As is disclosed in that patent, the base compositions for such glasses require the presence of silver, an alkali metal oxide, customarily Na.sub.2 O, fluoride, and at least one other halide selected from the group chloride, bromide, and iodide. The glasses are exposed to high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and ultraviolet radiations in the range of about 2800 A-3500 A. Where ultraviolet radiations comprise the actinic radiation, CeO.sub.2 will be included in the glass composition.
In the method described therein capable of producing a full spectrum of colors, the glass is initially irradiated with high energy or actinic radiations to develop a latent image therein. The length of this exposure and the flux thereof, i.e., the energy/unit area of the irradiation, determine the final color that will be exhibited by the glass. Thereafter, the glass is subjected to a heat treatment at a temperature between about the transformation range and the softening point thereof to precipitate colloidal silver particles in situ which perform as nuclei. In the case of a transparent colored glass, this heat treatment is undertaken only for so long as to effect precipitation of colloidal silver nuclei with the possible growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, normally, for example, NaF+(AgCl and/or AgBr and/or AgI).
The nucleated glass is subsequently cooled, commonly to room temperature but at least to a temperature 25.degree. C. below the strain point of the glass, and again exposed to high energy or actinic radiation. This second exposure develops the color, the hue of which had been determined through the previous exposure. Finally, the glass is reheated to a temperature between about the transformation range and the softening point of the glass to bring out the desired color.
The mechanism of color production was acknowledged as not being fully understood but it was postulated that the quantity of silver precipitated and the geometry of the precipitated particles, as well as perhaps the refractive index of the crystals developed, determined the colors manifested. Nevertheless, because the colors can be obtained with very minor quantities of silver, it was deduced that at least one of the following situations was present: (a) the presence of discrete colloidal particles of silver less than about 200 A in the smallest dimension; (b) the presence of metallic silver deposited within the alkali metal fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200 A in the smallest dimension; and (c) the presence of metallic silver deposited upon the surface of the microcrystals, the silver-coated portion thereof being less than about 200 A in the smallest dimension. The microcrystals are present in the glass in a concentration of at least about 0.005% by volume.
The patent further observed that consecutive or interrupted heat treatments, either subsequent to the initial irradiation to high energy or actinic radiation or following the second irradiation step, can act to intensify the final color produced. Accordingly, whereas the cause of this phenomenon was not comprehended, empirical evidence indicated that two or more heat treatments at temperatures between the transformation range and softening point of the glass can enhance and make the color resulting therefrom more vivid than is achieved through a single heat treatment of equal or longer duration.
As was mentioned above, the patent also noted that the identity of the color produced in the glass was dependent upon the duration and flux of the first exposure to high energy or actinic radiation. Thus, it was observed that the least quantum of exposure developed a green coloration followed by blue, violet, red, orange, and yellow as the exposure time and/or flux was increased.
U.S. Pat. No. 4,092,139 discloses an improvement upon the basic method for producing polychromatic glasses, as set forth in U.S. Pat. No. 4,017,318, supra, which shortens the time required for generating the desired colors and the colors, themselves, are frequently more vivid. The preferred embodiment of that inventive method involved four general steps:
(a) a glass article is formed having a composition included within the ranges described in U.S. Pat. No. 4,017,318;
(b) the glass article is exposed to high energy or actinic radiation for a sufficient period of time to develop a latent image therein;
(c) the glass article is removed from the high energy or actinic radiation and heated to a temperature between the transformation range and the softening point of the glass for a length of time sufficient to cause nucleation and growth of microcrystals of alkali metal fluoride containing at least one silver halide selected from the group of AgCl, AgBr, and AgI; and then
(d) the glass article is re-exposed to high energy or actinic radiation while at a temperature between about 200.degree.-410.degree. C. for a period of time adequate to cause metallic silver to be deposited as discrete colloidal particles less than about 200 A in the smallest dimension, and/or deposited upon the surface of said microcrystals, the silver-coated portion of the microcrystal being less than about 200 A in the smallest dimension; and/or deposited within said microcrystals, the silver-containing part of the microcrystal being less than about 200 A in the smallest dimension, said microcrystals having a concentration of at least 0.005% by volume.
Where desired, the first irradiation step may also be conducted at temperatures between about 200.degree.-410.degree. C. That technique does not appear to significantly improve the intensity of the final color in the glass, although it does have the advantage of reducing the time demanded for nucleation and incipient crystallization. However, when this first irradiation of the glass at elevated temperatures is prolonged for too extended a period of time, the glass will develop a permanent yellowish cast which obviously is unwanted where a spectrum of colors is desired.
As can be seen, both U.S. Pat. No. 4,017,318 and U.S. Pat. No. 4,092,139 require two exposures to high energy or actinic radiation to develop a spectrum of colors. Customarily, when practicing those inventions, the first irradiation treatment will typically be of relatively short duration, to more than several minutes. The second exposure, however, will commonly be for a much longer period, e.g., one hour or more, even when combined with the second heat treatment as disclosed in U.S. Pat. No. 4,092,139. Such practice is not only expensive due to the energy required for the long term exposure to high intensity radiation, but also limits the size and geometry of articles that can be treated.
U.S. Pat. No. 4,118,214 describes a further improvement upon the basic disclosure relating to polychromatic glasses, as set forth in U.S. Pat. No. 4,017,318, which alleviates those problems. Hence, the improvement lies in the finding that the second irradiation treatment with high energy or actinic radiation can be eliminated without sacrificing any portion of the spectrum of colors and the colors are equivalent in intensity to those produced according to the processes of the two above-mentioned patents. The inventive method contemplates firing the nucleated glass, i.e., the glass subjected to the initial irradiation step and heat treatment, in a gaseous reducing atmosphere at temperatures of at least 350.degree. C., but not above the strain point of the glass, and at gas pressures greater than ambient pressure. When temperatures above the strain point of the glass are employed, the color centers in the glass are thermally altered with the glass commonly taking on a permanent yellow coloration.
The method is operable with the glass compositions described in U.S. Pat. No. 4,017,318. The preferred compositions reported therein consist essentially, in weight percent on the oxide basis, of about 10-20% Na.sub.2 O, 0.0005-0.3% Ag, 1-4% F, an amount of at least one halide selected from the group of Cl, Br, and I at least sufficient to react stoichiometrically with the Ag, but not more than a total of 4%, and the remainder SiO.sub.2. Where ultraviolet radiation with wavelengths between about 2800 A-3500 A comprises the actinic radiation, about 0.01-0.2% CeO.sub.2 will be included in the composition. Furthermore, Sb.sub.2 O.sub.3 and/or SnO can be utilized as thermoreducing agents in the stated proportions of about 0.1-1% Sb.sub.2 O.sub.3 and/or about 0.01-1% SnO, with the total Sb.sub.2 O.sub.3 +SnO not exceeding about 1%. Up to 18% ZnO and up to 10% Al.sub.2 O.sub.3 are stated to be useful additions.
In further delineating the character of the glasses, each of the three patents discussed above noted that, in the case of transparent glasses, the concentration of microcrystals therein will be maintained below about 0.1% by volume and the size thereof will be no larger than about 0.1 micron in diameter. Finally, to insure the preparation of transparent glasses, the silver content will customarily be held below about 0.1% by weight, the fluoride level will not exceed about 3% by weight, and the total of the remaining halides will be maintained below about 2% by weight.
An atmosphere of hydrogen constitutes the most effective reducing environment in terms of production speed. Reducing atmospheres less hazardous than hydrogen alone are well recognized in the art, e.g., cracked ammonia, mixtures of CO and CO.sub.2, and various blends of N.sub.2 and H.sub.2, marketed under the term, forming gas. These environments are also effective but require longer firing periods.
The rate of hydrogen permeation into glass is dependent upon the exposure temperature and the pressure of the hydrogen-containing atmospheres. Consequently, the diffusion rate will be increased when the temperature at which the glass is subjected to the hydrogen environment is raised and/or the pressure of the hydrogen atmosphere is increased. Furthermore, the use of wet reducing gas, e.g., forming gas that has been passed through liquid water or otherwise combined with water vapor, may be more effective than gas in the dry state.
To recapitulate, the optimum temperature for the thermoreducing treatment will desirably be as high as possible to maximize hydrogen diffusion into the glass, but below the strain point of the glass. Where pure hydrogen is utilized as the reducing environment, treatment temperatures between about 425.degree.-475.degree. C. are preferred. A temperature of about 500.degree. C. is deemed to constitute a practical maximum to permit careful control of color production. The inventive method enables the development of very vivid colors in thin layers of glass, the depth of the layer depending upon the duration of the hydrogen firing.
U.S. Pat. No. 4,017,318 describes attempts to produce color photographs in the glass. Although, as is illustrated therein, faithful images of the objects to be photographed were secured, the colors of the original objects were not successfully reproduced in the glass. Hence, while achieving high optical resolution in the glass, utility in the color photography market demanded a matching and/or improvement upon the colors.
One color photographic process employing glass plates which has been marketed commercially, but on a relatively small scale, has been designated the "Screen-Plate Process". This process involves mechanically laying down a patterned mosaic array or screen of large numbers of very tiny red, green, and blue filters on a glass plate, those filters conventionally consisting of coatings containing organic dyes. This array is placed directly in contact with a panchromatic film of the type employed in black-and-white photography. The camera exposure is made through the array. After exposure, the film is developed by reversal to yield a positive silver image or, alternatively, a negative is first prepared and a positive then made therefrom. This combination of polychromatic filter plate and the black-and-white emulsion backing is the completed color transparency picture which is viewed directly or projected onto a screen by passing light back through the silver image in combination with the original plate, or one having an identical array of color filters. The color filters control the spectral characteristics of the transmitted light and the relative intensities are a function of the densities of the silver image. The color filters are of such size that the light transmitted through them visually fuses to yield additive color mixtures.